There is an increasing demand for many plastic products used in packaging to be biodegradable. Trays in biscuit and chocolate packages are one example. Starch films have been proposed as biodegradable alternatives for some time. U.S. Pat. No. 3,949,145 proposed a starch/polyvinyl alcohol/glycerol composition for use as a biodegradable agricultural mulch sheet.
Difficulties have been encountered in producing starch based polymers particularly by hot melt extrusion. The molecular structure of the starch is adversely affected by the shear stresses and temperature conditions needed to plasticise the starch and pass it through the extrusion die. For most products foaming has to be avoided and this generally requires attention because of the water content of the starch. Foaming has been avoided by degassing the melt prior to exiting the die as suggested in U.S. Pat. Nos. 5,314,754 and 5,316,578. The latter patent also avoids adding water to the starch. As explained in U.S. Pat. No. 5,569,692 by not drying starch and avoiding the addition of water the starch can be processed at temperatures between 120° C. and 170° C. because the water bound to the starch does not generate a vapour pressure such as to require high pressures. Another approach to improving the melt processability of starch is to provide an additive as in U.S. Pat. No. 5,362,777 which reduces the melting point of the starch. The additive is selected from dimethyl sulfoxide, a selection of polyols and amino or amide compounds.
In order to produce starch polymers for particular applications they have been blended with a range of other polymers. Biodegradable blown films are disclosed in U.S. Pat. No. 5,322,866 which blends raw starch, polyvinyl alcohol and talc with glycerol and water. U.S. Pat. No. 5,449,708 discloses compositions of starch ethylene acrylic acid and a salt of stearic acid plus a glycerol based lubricant. Flexible and clear transparent sheets are disclosed in U.S. Pat. No. 5,374,304. These are composed of a high amylose starch and a glycerol plasticizer. The use of starch in conjunction with high amylose or modified starches has also been proposed. U.S. Pat. Nos. 5,314,754, and 5,316,578 both suggest the use of modified starches including hydroxypropyl substituted starches. Hydroxypropylation reportedly increases elongation at break and burst strength and improved resilience in the polymers. Although the efficacy of these special and modified starches is recognised, their cost inhibits the commercial acceptability of the products made from them.
It is an object of this invention to provide a biodegradable polymer which can be processed and thermoformed into sheet and shaped products without difficulty and have acceptable properties for its intended uses.